1. Field of the Invention
The present invention relates generally to the fabrication of glass preforms and more particularly, to a method and apparatus for forming an elongated glass optical preform used to make optical fibers.
2. Description of the Prior Art
Glass fibers for optical communication are made from high purity, silica-based glass fibers drawn from glass preforms. Various glass deposition techniques are known for producing optical preforms which are then drawn into optical fibers. Some of these techniques, including vapor axial deposition (VAD) and outside vapor deposition (OVD), are based on flame combustion wherein reactants are fed together with combusting gases through a burner onto the growing preform. A porous glass preform is thus fabricated, which is then consolidated into a solid glass preform apt to being subsequently drawn into an optical fiber. According to other deposition techniques, such as modified chemical vapor deposition (MCVD) or plasma chemical vapor deposition (PCVD), the reactants are fed into a preformed glass tube, which is heated from the outside with a conventional combustion burner (MCVD) or with a plasma generating torch (PCVD); silica particles are deposited within the tube, thus forming the preform which is then drawn into an optical fiber.
The known combustion methods are based on the use of a flame burner to provide the energy for the reaction. Examples of the VAD process are disclosed, for example, in U.S. Pat. Nos. 5,597,398 and 4,915,717. Examples of the OVD process are disclosed in U.S. Pat. Nos. 3,806,570, 4,204,851, 4,596,589, and 4,810,276.
In the known combustion processes using burners (using either H2/O2 or CH4/O2 as combustion mixtures), silica particles are produced by a main oxidation reaction (1) of silica precursors and by a secondary reaction (2) involving flame hydrolysis of a silica precursor, according to the following mechanisms:
2H2+O2xe2x86x922H2O or CH4+2O2xe2x86x922H2O+CO2(combustion)
SiCl4+O2xe2x86x92SiO2+2 Cl2xe2x80x83xe2x80x83(1)
SiCl4+2H2Oxe2x86x92SiO2+4 HClxe2x80x83xe2x80x83(2)
U.S. Pat. No. 4,414,164 discloses a low temperature hydrolysis process for fabricating silica preforms, which avoids the use of a flame burner, wherein silicon tetrachloride is reacted directly with water, according to the above reaction (2), in order to produce a silica preform.
Further techniques for producing a preform using flame-free techniques based on the above hydrolysis reaction are also known, as disclosed, for example, in U.S. Pat. Nos. 4,564,378, 4,597,983, 4,650,693, and 4,735,643. For instance, U.S. Pat. No. 4,564,378 discloses the reaction of a glass forming chloride gas (e.g. SiCl4) with water. The formed silica particles are deposited onto a target contained into a reaction chamber, said target being cooled to a temperature of 20xc2x0 C. to 800xc2x0 C. (preferably 20xc2x0), while the surrounding reaction chamber is heated at a higher temperature, preferably about 1000xc2x0 C. U.S. Pat. No. 4,597,983 teaches the use of an aerosol-free gas stream to envelope and convey an aerosol stream, formed by reaction of silica and water, to be deposited e.g. onto a bare like body. U.S. Pat. No. 4,650,693 teaches to heat said aerosol-free confining stream to a temperature which is higher than the temperature of the confined aerosol stream, in order to increase the confining effect of the aerosol-free gas. U.S. Pat. No. 4,735,643 teaches to add to the reacting mixture, e.g. gaseous SiCl4 and water, at least one gas product of at least one of the gas phase reactants, for instance HCl, in order to avoid undesirable deposition of material at the point of entry of the reactants into the reaction chamber.
Although the above methods avoid some problems associated with other techniques where a flame hydrolysis is employed, the applicant has observed various drawbacks connected with these techniques.
For instance, it has been observed that too low temperatures of the reactants (e.g. silicium tetrachloride and water) may cause an incomplete hydrolysis reaction, and hydrated polymeric products may be generated in the final preform. These intermediate products may impair the quality of the final preform and cause nozzle obstruction.
On the other side, as disclosed by U.S. Pat. No. 4,735,643, high temperature of the reactant gases at the reactor inlet causes unwanted material deposition at the inlet.
The applicant has however noticed that the solution adopted in said patent, i.e. introduction of a reaction product (HCl, in the specific) into the reaction mixture entering the reaction chamber, causes an increase of the volume of gases entering the reaction chamber, with part of said volume not being at disposal for the glass forming reaction.
In addition, use of a confining gas stream around the reactants results in increased size and complexity of the apparatus.
The present invention relates to a method based on a hydrolysis reaction for producing an optical preform, wherein an increasing temperature gradient is provided between the inlet and the outlet zone of the reactor. In addition, according to the method of the present invention also the temperature of the target preform and/or the temperature of the reactants at the inlet of the reaction chamber is suitably controlled.
The hydrolysis reaction on which the method of the present invention is based is typically a flame-free hydrolysis, i.e. a reaction in which the process temperature can be precisely controlled, as opposed to conventional flame-hydrolysis wherein the temperature is hardly controllable within predetermined process parameters. Flame-free hydrolysis is thus generally performed in the substantial absence of combustibles, such as CH4 or H2.
A first aspect of the present invention thus relates to a method for manufacturing a glass preform by depositing an aerosol stream of glass particles onto a target, which comprises:
supplying a first gaseous or vapor phase composition disposed to provide a hydrolyzable glass precursor to an inlet zone of a reaction chamber;
supplying water as a second gaseous or vapor phase composition to said inlet zone of the reaction chamber;
reacting the water and the first gaseous or vapor phase composition in the reaction chamber to form an aerosol of glass particles;
directing the aerosol along said chamber and through an outlet of said chamber onto a target on which the preform is formed; and
depositing the aerosol on the target,
characterized in that a temperature gradient is provided inside of said chamber, said temperature gradient being such that the temperature increases from said inlet zone to said outlet of the reaction chamber.
According to a preferred embodiment, a difference of temperature of at least about 100xc2x0 C. is provided from said inlet zone to said outlet of the reaction chamber, said difference of temperature preferably being of about 300xc2x0 C. and up to about 800xc2x0 C.
According to a further preferred embodiment, the temperature of the aerosol stream being directed through the reaction chamber increases from about 700xc2x0 C. at the inlet to about 1200xc2x0 C at. the outlet of said chamber.
According to an embodiment of the present invention, the water and the first gaseous or vapor phase composition are reacted in the substantial absence of an unreactive carrier gas. Preferably, the first and the second gaseous or vapor phase compositions are obtained by separately heating, under pressure, the first and second compositions, each contained as pure liquid in a respective supply tank.
According to a preferred embodiment of the present invention, the first and the second gaseous or vapor phase compositions are supplied at a predetermined temperature to the chamber, said predetermined temperature being a temperature at which the hydrolysis reaction between the two compositions is substantially incomplete. With the expression xe2x80x9csubstantially incomplete hydrolysis reaction,xe2x80x9d it is intended that the dimension of the silica particles produced by the reaction is sufficiently small in order to allow being transported by the gas stream without giving rise to unwanted deposition of material at the inlet of the reaction chamber, as observed in prior-an processes. In particular, said predetermined temperature is about 800xc2x0 C. or lower, preferably from about 600xc2x0 C. to about 750xc2x0 C., a temperature of about 700xc2x0 C. being particularly preferred.
According to a preferred embodiment, the temperature of the target preform on which glass particles are deposited is higher than about 700xc2x0 C. Preferably, said target preform is maintained at a temperature which is lower than the temperature of the aerosol stream impacting on said preform. Preferably, the temperature of the target preform is at least 100xc2x0 C. less than the temperature of the aerosol stream impacting on said preform.
A further aspect of the present invention relates to an apparatus for forming an elongated glass optical preform comprising:
a target onto which glass is deposited to form a preform;
an injection system for supplying a first gaseous or vapor phase composition and gaseous or vapor phase water to an inlet zone of a reaction chamber;
a reaction chamber in which the gaseous or vapor phase water and the first gaseous or vapor phase composition are reacted to form an aerosol of glass, said reaction chamber being provided with an outlet through which the aerosol of glass is directed toward the target;
a heating system associated with said reaction chamber, said heating system providing a temperature gradient inside said chamber, said temperature gradient being such that the temperature increases from said inlet zone to said outlet of the reaction chamber.
Preferably, said reaction chamber has a cross section which is convergent from an inlet zone to an outlet zone of the reactants.
In the present description, the expression xe2x80x9chydrolyzable glass precursorxe2x80x9d is intended to indicate any suitable component or mixture of components able to react with water in order to create a glass. As the typical glass component is silica, a hydrolyzable precursor is thus typically a silicon compound (for instance silicon tetrachloride), which can be subjected to the hydrolysis reaction alone or in admixture with other glass precursors, such as the so-called doping agents, which comprise hydrolyzable compounds of Germanium, Boron, Phosphorus, Aluminum, Titanium, Zirconium and Fluorine.
The accompanying drawings, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention.